TORONTO - After months of negative news about the state of polio eradication efforts in Pakistan, there appears to be a sign of progress, new data suggest.

One of the two types of polio viruses circulating there may be on the verge of petering out, Dr. Steven Wassilak, of the U.S. Centers for Disease Control, reported Tuesday at a medical conference in Atlanta, Ga.

There hasn't been a case of polio caused by the Type 3 virus since April, Wassilak said at the annual meeting of the American Society for Tropical Medicine and Hygiene.

While a seven-month gap isn't long enough to say that Type 3 viruses have been eliminated, there is another reason to hope transmission of Type 3 may be dying out in Pakistan.

Scientists at the CDC study the molecular makeup of polio viruses and they say the Type 3 viruses are down to one remaining chain of transmission. In some countries that have successfully driven out polio, a reduction of the number of transmission chains has preceded the disappearance of virus types.

"The Type 3 (virus) is approaching a point where it's never been before. It's a single family that's been in circulation over the last year and a half, in an isolated place," Wassilak said in an interview late last week.

The area where the most recent Type 3 case was reported is in a remote part of Pakistan, where polio vaccination teams are often refused entry. Surveillance for cases in that region isn't perfect, Wassilak admitted. But with no sign that Type 3 viruses are causing cases outside of that area, he and others are expressing cautious optimism.

"(It) gives us promise that we're approaching the end of transmission (of Type 3)," said Wassilak, the CDC's team lead for science, innovation and research for the polio response in its Emergency Operations Center.

There are three strains or serotypes of polio viruses, called Type 1, Type 2 and Type 3. Type 2 viruses have already been eradicated; the last case of paralysis caused by a Type 2 virus occurred in India in 1999.

This year Type 3 cases are down to incredibly low numbers. Of the three countries where polio is still considered endemic (meaning transmission of the virus has never been stopped), Afghanistan has had no Type 3 cases, Pakistan has reported two and Nigeria has had 18.

For a time, Pakistan seemed poised to be perhaps the last country on earth to rid itself of polio. Resistance to polio vaccine among some Muslim families poses a tough challenge for the eradication program. Vaccination teams are barred from entering some communities and are at times under threat.

Earlier this year a volunteer was shot and killed while making vaccination rounds. In a separate incident, the child of a polio worker was kidnapped, though later released.

Another presentation to the conference suggests overcoming the obstacles to vaccination in Pakistan won't be easy. Dr. Anita Zaidi, who serves on the country's national immunization technical advisory group, says vaccine refusal is deeply entrenched in some families.

Despite these very real problems, the molecular science of what is going with polio viruses in Pakistan suggest efforts are paying off, CDC officials said.

"Type 3 looks like it is hanging on by a single chain of transmission in a very localized area," said Dr. Olen Kew, associate director for global laboratory science at the CDC's National Center for Immunization and Respiratory Diseases.

"There are some difficulties in Pakistan that we all appreciate. But they're making progress."

The dwindling of Type 3 cases in Pakistan suggests the virus is under pressure from vaccination efforts, agreed Dr. Bruce Aylward, the World Health Organization's point person for polio eradication. Aylward, a Canadian, is assistant director-general for polio, emergencies and country collaboration.

But Aylward insisted it's too soon to suggest Type 3 viruses are gone from Pakistan. For one thing, he said, it is currently high season for polio transmission. Viruses or chains of viruses that have gone to ground often pop up during high season, he said.

And Type 3 viruses can go unseen for a time, probably because of the way the viruses attacks.

With polio, only a small portion of infections lead to paralysis. It's estimated that for every child polio paralyzes, another 200 will have been infected and survived without disability. But in fact, with Type 3 viruses, the rate is probably more like one case of paralysis for every 1,000 infections, Aylward said, which explains how the virus can spread, unseen, for periods of time.

The most recent Type 3 case in Pakistan was found in a part of the Federally Administered Tribal Areas, in the Northwest of the country along the border with Afghanistan. The region is not under the control of Pakistan's central government.

Resistance to polio vaccination there is high. In fact, in September, polio campaign workers were allowed for the first time in three years to vaccinate children in Bara Teshsil, the sub-district where the most recent Type 3 case occurred.

"There's clear evidence that there's good pressure on the Pakistan (Type 3) virus this year, much more than last year," Aylward said. "Will that pay a dividend in knocking out a serotype? That would be fabulous. Just premature to say it, that's all."

The campaign to rid the world of polio was launched in 1988 by four founding partners — the WHO, Rotary International, the CDC and UNICEF. In more recent years, the Bill and Melinda Gates Foundation has become an active and generous partner in the work.

To date this year there have been only 181 cases of paralytic polio worldwide, in four countries: Nigeria, Pakistan, Afghanistan and Chad. The Global Polio Eradication Initiative hopes to be able to declare the world polio free in 2018.